1. Field of the Invention
The present patent relates to a drain pump for flash memory, and more specifically to a drain pump for flash memory in which an output voltage is varied depending on a number of bits to be programmed.
In a code flash memory, when a stacked gate cell is employed, a voltage of about 9V is supplied to a gate and a voltage of about 5V is supplied to a drain in order to execute a program, whereby hot carriers are generated, and stored in a floating gate.
2. Discussion of Related Art
Generally, when a program is operated in a word mode (for example, X16), all 16 bits may be concurrently programmed. At that time, a current flowing from a drain to a source is significantly larger than that in the case of programming a fewer number of bits. Actually, since electric charges are stored in hot carriers generated by the current, if a process is restrictive, it is not possible to decrease the current in order to reduce time for improving program efficiency and increasing a suitable threshold voltage. When a current per bit is 500 μA and all bits are programmed in the word mode, a current of 8 mA flows. At that time, a drain pump for generating a voltage of 5V to be supplied to the drain cannot know the number of bits to be currently programmed, the drain pump always supplies a bias voltage to cells with ability of supplying a current of 800 mA or more. Because the number of bits to be programmed is 1 to 16 bits, but a constant voltage is supplied regardless of the number of bits, there is a problem that current dissipation is increased.